U.S. patent number 3,770,928 [Application Number 05/211,926] was granted by the patent office on 1973-11-06 for reliable solid state induction cooking appliance with control logic.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr., William P. Kornrumf.
United States Patent |
3,770,928 |
Kornrumf , et al. |
November 6, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
RELIABLE SOLID STATE INDUCTION COOKING APPLIANCE WITH CONTROL
LOGIC
Abstract
A smooth-top cooking appliance for inductively heating cooking
utensils comprises a flat induction heating coil driven at an
ultrasonic frequency by a solid state inverter. The control circuit
for the inverter is suitable for fabrication as an integrated
circuit and includes, in addition to turn-on circuitry, protection
circuits to assure reliable and automatic operation under abnormal
circuit conditions such as overvoltages and low input voltages
tending to cause device and power circuit failures. The control
circuit also includes a utensil presence detection circuit to
assure operation under load and no-load conditions. Voltage
responsive sensors such as Zener diodes sense the appropriate
voltages at selected points on the power circuit and modify the
operation of the control circuit, preferably by over-riding and
inhibiting the turn-on circuitry. Disclosed with regard to a
one-thyristor, variable frequency series resonant inverter with an
added maximum frequency control, the protection circuit technique
is applicable to inverters generally.
Inventors: |
Kornrumf; William P.
(Schenectady, NY), Harnden, Jr.; John D. (Schenectady,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
22788836 |
Appl.
No.: |
05/211,926 |
Filed: |
December 27, 1971 |
Current U.S.
Class: |
219/626; 363/57;
219/668 |
Current CPC
Class: |
H05B
6/062 (20130101); H02M 7/525 (20130101); H02M
5/45 (20130101); H02M 7/523 (20130101) |
Current International
Class: |
H02M
5/00 (20060101); H05B 6/06 (20060101); H02M
7/505 (20060101); H02M 7/525 (20060101); H02M
7/523 (20060101); H02M 5/45 (20060101); H05b
005/04 () |
Field of
Search: |
;219/10.49,10.75,10.77,10.79 ;321/11,14 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Reynolds; Bruce A.
Claims
What I claim as new and desire to secure by Letters Patent of the
United States is:
1. A reliable solid state cooking appliance for inductively heating
a cooking utensil comprising
an induction heating coil mounted adjacent a substantially unbroken
non-metallic utensil support and producing an alternating magnetic
field,
a static power conversion circuit including a power circuit
controlled by solid state power device means for generating an
ultrasonic frequency wave that drives said induction heating coil,
and
a control circuit operating at signal level and connected to
selected points on said power circuit, said control circuit
comprising a turn-on circuit for selecting the intervals of
conduction of said solid state power device means, and further
comprising protection circuit means for detecting predetermined
abnormal power circuit conditions and modifying the operation of
said control circuit to obtain reliable and automatic operation of
said static power conversion circuit.
2. A cooking appliance according to claim 1 wherein said protection
circuit means modifies the operation of said turn-on circuit.
3. A cooking appliance according to claim 1 wherein said protection
circuit means comprises an overvoltage detection circuit that
monitors the voltage at a terminal of said solid state power device
means and is operative in response to the sensing of a voltage that
rises above a predetermined level.
4. A cooking appliance according to claim 1 wherein said protection
circuit means includes a low input voltage detection circuit that
monitors the input voltage to said power circuit and is operative
in response to the sensing of a input voltage below a predetermined
level.
5. A cooking appliance according to claim 4 wherein said low input
voltage detection circuit is operative during start-up and
shut-down of said static power conversion circuit when power is
supplied to and removed from said circuit.
6. A cooking appliance according to claim 1 wherein said turn-on
circuit operates at a variable repetition rate to vary the
frequency of the ultrasonic wave generated by said static power
conversion circuit and thereby adjust the power supplied to the
cooking utensil.
7. A cooking appliance according to claim 6 wherein said protection
circuit means also includes a maximum frequency control circuit for
limiting the maximum repetition rate of said turn-on circuit.
8. A cooking appliance according to claim 1 wherein said control
circuit further includes a utensil presence detection circuit that
senses the absence of a cooking utensil coupled with said induction
heating coil and modifies the operation of said control
circuit.
9. A cooking appliance according to claim 8 wherein said utensil
presence detection circuit modifies the operation of said turn-on
circuit means.
10. A reliable solid state cooking appliance for inductively
heating a cooking utensil comprising
an induction heating coil mounted adjacent a substantially unbroken
non-metallic utensil support and producing an alternating magnetic
field,
a static power conversion circuit including a power circuit
controlled by solid state power device means for generating an
ultrasonic frequency wave that drives said induction heating coil,
and
a control circuit operating at signal level and including a turn-on
circuit for selecting the intervals of conduction of said solid
state power device means, and further including utensil presence
detection circuit means for sensing a parameter at a selected point
on said power circuit during a predetermined time interval and
modifying the operation of said control circuit in response to a
sensed parameter indicative of the absence of a cooking utensil
coupled with said induction heating coil.
11. A cooking appliance according to claim 10 wherein said utensil
presence detection circuit means comprises a voltage responsive
sensor connected between said selected point on said power circuit
and said control circuit, and override circuit means coupled with
said voltage responsive sensor for modifying the operation of said
turn-on circuit in response to the sensing of a predetermined
voltage level.
12. A cooking appliance according to claim 11 wherein said utensil
presence detection circuit means further comprises enabling circuit
means responsive to the voltage at said selected point on said
power circuit that enables said override circuit means only during
said predetermined time interval.
13. A cooking appliance according to claim 12 wherein said voltage
responsive sensor comprises at least one Zener diode.
14. A reliable solid state cooking appliance for inductively
heating a cooking utensil comprising
an induction heating coil mounted adjacent a substantially unbroken
non-metallic utensil support plate and producing an alternating
magnetic field,
a static power conversion circuit including a unidirectional input
voltage supply and an inverter power circuit controlled by solid
state power device means for generating an ultrasonic frequency
wave that drives said induction heating coil, and
a signal level control circuit connected to selected points on said
power circuit,
said control circuit comprising turn-on circuit means for selecting
the intervals of conduction of said solid state power device means,
protection circuit means for detecting predetermined abnormal power
circuit conditions and modifying the operation of said control
circuit, and utensil presence detection circuit means for detecting
the absence of a cooking utensil on said support coupled with said
induction heating coil and for modifying the operation of said
control circuit, to thereby achieve reliable and automatic
operation of said static power conversion circuit.
15. A cooking appliance according to claim 14 wherein said turn-on
circuit means has a user adjustable variable repetition rate to
vary the frequency of said ultrasonic frequency wave and therefore
the power supplied to the cooking utensil.
16. A cooking appliance according to claim 15 wherein said
protection circuit means includes an overvoltage detection circuit
that senses a predetermined voltage at a terminal of said solid
state power device means and is operative to reduce the repetition
rate of said turn-on circuit means.
17. A cooking appliance according to claim 16 wherein said
overvoltage detection circuit comprises a low pass filter so that
the circuit is non-responsive to high frequency transient
overvoltages.
18. A cooking appliance according to claim 15 wherein said
protection circuit means includes a low input voltage detection
circuit that senses a predetermined low input voltage and is
operative to reduce the repetition rate of said turn-on circuit
means.
19. A cooking appliance according to claim 18 wherein said low
input voltage detection circuit further includes a time delay
circuit and an inhibit circuit coupled therewith that reduces the
repetition rate of said turn-on circuit means to zero after a
predetermined time delay.
20. A cooking appliance according to claim 15 wherein said utensil
presence detection circuit means is operative to reduce the
repetition rate of said turn-on circuit in response to the sensing
of a power circuit parameter indicative of the absence of a coupled
cooking utensil.
21. A reliable solid state cooking appliance for inductively
heating a cooking utensil comprising
a nominally flat induction heating coil mounted adjacent a
substantially unbroken non-metallic utensil support plate and
producing an alternating magnetic field,
a static power conversion circuit comprising a unidirectional input
voltage supply and a variable ultrasonic output frequency series
resonant inverter power circuit controlled by a thyristor device
that drives said induction heating coil, and
a firing control circuit for said thyristor device energized by low
voltage power supply means and including a variable repetition rate
firing signal circuit for supplying firing signals to said
thyristor device at a selected repetition rate to obtain the
desired heating level,
said firing control circuit further including protection circuit
means comprising overvoltage detection circuit means for sensing
the voltage at one terminal of said thyristor device and overriding
said firing signal circuit in response to an overvoltage condition,
and low input voltage detection circuit means for sensing the input
voltage to said inverter circuit and overriding said firing signal
circuit in response to a low input voltage condition.
22. A cooking appliance according to claim 21 wherein said firing
signal circuit comprises a voltage controlled pulse generator whose
repetition rate is determined by the voltage on a control
capacitor,
said control capacitor being connected in series with a user
adjustable resistive component between the terminals of said low
voltage power supply means to set said control capacitor voltage to
obtain the desired inverter output frequency and heating level.
23. A cooking appliance according to claim 22 wherein said
protection circuit means further includes a maximum output
frequency control circuit comprising a voltage clamping circuit
connected across the terminals of said control capacitor.
24. A cooking appliance according to claim 22 wherein said
overvoltage detection circuit means comprises a voltage responsive
sensor connected between a terminal of said thyristor device and
said firing control circuit, and
a discharge circuit connected across the terminals of said control
capacitor and coupled to said voltage responsive sensor to be
actuated by the sensing of an overvoltage.
25. A cooking appliance according to claim 24 wherein said voltage
responsive sensor comprises at least one Zener diode.
26. A cooking appliance according to claim 22 wherein said low
input voltage detection circuit means comprises a voltage
responsive sensor connected between a terminal of said input
voltage source and said firing control circuit, and
a voltage clamping circuit connected across the terminals of said
control capacitor and coupled to said voltage responsive sensor to
be actuated by the sensing of a low input voltage.
27. A cooking appliance according to claim 26 wherein said low
input voltage detection circuit means further comprises inhibit
circuit means responsive to the sensing of a low input voltage for
inhibiting the production of firing signals by said voltage
controlled pulse generator.
28. A cooking appliance according to claim 22 wherein said firing
control circuit further includes a utensil presence detection
circuit,
said utensil presence detection circuit comprising a voltage
responsive sensor connected between a terminal of said thyristor
device and said firing control circuit, a discharge circuit
connected across the terminals of said control capacitor and
coupled with said voltage responsive sensor to be actuated by the
sensing of a predetermined voltage, and discharge circuit enabling
means having a connection to a terminal of said thyristor device
for enabling said discharge circuit only during a predetermined
time interval following reapplication of forward voltage to said
thyristor device.
Description
BACKGROUND OF THE INVENTION
This invention relates to solid state cooking appliances based on
induction heating, and more particularly to improved control
circuits for achieving reliable and convenient operation of these
appliances so as to be suitable for mass usage.
The application of induction heating to the cooking of food has
been known generally for a number of years. The basic mechanism is
that the alternating magnetic field produced by an induction
heating coil is coupled across a gap and the utensil support with
the utensil bottom, which acts as a single turn secondary winding.
Theoretically the process is efficient since heat is generated only
in the metallic utensil where it is wanted, and none is lost to the
surrounding atmosphere. Nevertheless, previous equipment for
induction cooking in general was bulky and expensive and not
potentially competitive with the common gas range and conventional
electrical range based on resistance heating.
Solid state induction cooking appliances operating at ultrasonic
frequencies of 18 kHz and above make possible a significant
reduction in cost and size and overcome other deficiencies of the
prior art equipment. An economical appliance with simplified power
circuits is described in copending application Ser. No. 200,424,
filed Nov. 19, 1971 by the present inventors, and assigned to the
same assignee as this invention. This appliance is suitable for an
induction surface unit in an electric range or a portable
counter-top warming or cooking appliance. Basically it comprises a
flat spiral air-core or ferromagnetic-core induction heating coil
mounted beneath a smooth, unbroken non-metallic cooktop surface. A
static power conversion circuit typically formed by a rectifier and
an inverter generates an ultrasonic frequency wave for driving the
induction heating coil. The simplified inverter power circuit uses
only one thyristor or transistor and therefore only one firing or
base drive circuit, and uses the induction heating coil in a dual
function as the inductance in a series or parallel resonant circuit
as well as to couple power to the utensil.
Another essential feature of a successful induction cooking
surface, in addition to inexpensive power circuits, is that it be
reliable, safe, and convenient for use by the ordinary technically
unskilled person such as a chef or housewife. Inverters require
protection to prevent malfunctioning and failure due to abnormal
circuit conditions including overcurrents and overvoltages, and
this protection is especially needed for semiconductor components.
The problem is compounded by the fact that the utensil losses are
the inverter load and that the reflected inductance of the utensil
changes the inverter's electrical parameters. There are severe load
requirements if the unit is to be operable with a variety of
ordinary pots and pans of different sizes and materials, under load
and no-load conditions with the utensil on the unit or removed from
it, and with containers the unit was not designed to heat since
this is beyond the control of the manufacturer. The user will not
be content to replace fuses, reset circuit breakers or even restart
the unit in the event of conditions arising which have not been
anticipated in the circuit design. Furthermore, it is not desirable
to implement these functions by the addition of components to the
power circuit itself according to the prior art teachings since
these components conduct power currents and add disproportionately
to the cost. Automatic and safe operation by the consumer calls for
new approaches to these essential circuit functions.
SUMMARY OF THE INVENTION
The solid state cooking appliance to which the invention is
applicable comprises an induction heating coil that is mounted
adjacent to a substantially unbroken non-metallic utensil support
and produces an alternating magnetic field for coupling power to a
cooking utensil. A static power conversion circuit typically
comprising a full wave rectifier and a solid state inverter power
circuit generates an ultransonic frequency wave for driving the
induction heating coil. A control circuit for the power converter
operates at signal level and is designed to be manufacturable as a
hybrid or monolithic integrated circuit. The control circuit
conventionally includes a turn-on circuit for selecting the
intervals of conduction of the solid state power device or devices
controlling the operation of the power circuit. The innovation is
made that the control circuit also incorporates features such as
protection circuits and an optional utensil presence detection
circuit to obtain reliable and automatic operation of the cooking
appliance under a variety of load and no-load conditions and under
abnormal power circuit conditions such as overvoltages and low
input voltages that would if uncorrected tend to cause malfunction
or failure of the power device or other components of the power
circuit. This is accomplished preferably by overriding the normal
operation of the turn-on circuit. The absence of a cooking utensil
and the existence of abnormal power circuit conditions are sensed
preferably by means of voltage responsive sensors such as Zener
diodes connected between appropriate points on the power circuit
and the signal level control circuit. In this way the power circuit
remains simple since the required sensors and protective circuits
do not conduct power currents. Furthermore, the utensil presence
detection circuit does not require physical contact with the
cooking utensil and is compatible with the important unbroken
utensil support feature of the cooking appliance.
The invention is described by way of illustration with regard to a
power circuit in the form of a variable frequency series resonant
inverter employing a thyristor and inverse-parallel diode as the
solid state power devices. Adjusting the inverter output frequency
between about 18-40 kHz varies the power coupled to the cooking
utensil and therefore the cooking temperature. Only one firing
control circuit is required, and comprises a variable repetition
rate voltage controlled pulse generator controlled by the voltage
level on a control capacitor. A maximum frequency control can be
provided if desired and is operative to limit the repetition rate
of the firing pulse generator. This and the other protection
circuits ensure sufficient commutating energy and turn-off time for
the tyristor, and also control transients during start-up and
shut-down and under low input voltage conditions. The protection
circuits reduce the firing pulse repetition rate by clamping or
discharging the control capacitor, and also by inhibiting the pulse
generator. An overvoltage detection circuit senses the voltage at a
terminal of the thyristor and is operative when the voltage rises
above a predetermined maximum. A low input voltage detection
circuit is operative in response to the sensing of a low voltage at
the rectifier terminals, and includes an inhibit circuit for
completely stopping the production of firing pulses after a
predetermined time delay. The circuit also operates during start-up
and shut-down when power is supplied to and removed from the
circuit.
These protective circuit features are useful with inverter circuits
in general and are not limited to power converters for solid state
cooking appliances. Another control circuit feature specifically
needed for cooking appliances, however, is the utensil presence
detection circuit for sensing a selected power circuit parameter
indicative of the absence of a cooking utensil coupled with the
induction heating coil and modifying the operation of the control
circuit. In the series resonant inverter previously mentioned, the
reapplied forward voltage at the thyristor terminal is
characteristically initially higher in the unloaded case than in
the loaded case. The sensing of the absence of the cooking utensil
results in the lowering of the pulse generator repetition rate to
the audio level so that the user is aware the unit is turned
on.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a solid state converter for supplying
ultrasonic frequency power to a flat spiral induction heating coil,
shown in plan view, in an induction cooking appliance constructed
in accordance with the invention;
FIG. 2 is a diagrammatic cross-sectional view showing the relation
of the induction heating coil to the utensil support and cooking
utensil;
FIG. 3 is a fragmentary perspective view of an electric range with
a smooth utensil supporting top surface;
FIG. 4 is a schematic circuit diagram of a static power conversion
circuit comprising a rectifier and a one-thyristor series resonant
inverter, showing in block diagram form a control circuit that
provides adjustment of the cooking temperature and incorporates
automatic operation and protection features;
FIG. 5 is a simplified equivalent electrical circuit diagram of the
induction heating coil and cooking utensil load;
FIGS. 6a and 6b are waveform diagrams of the induction coil current
and commutating capacitor voltage for the converter of FIG. 4,
showing in each diagram the waveforms at two different inverter
output frequencies;
FIG. 6c is a waveform diagram of the forward voltage applied to the
thyristor of FIG. 4 under load and no-load conditions, showing for
reference purposes the damped sinusoidal induction coil current;
and
FIG. 7 is a detailed schematic circuit diagram of the firing
control circuit illustrated in block diagram form in FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The induction cooking appliance shown in FIGS. 1-3 will be
described with regard to an induction surface unit in an electric
range, but essentially the same mechanical structure and circuitry
in a lower power version is also suitable for a portable
counter-top food cooking or warming appliance. The static power
conversion circuit indicated generally at 12 is preferably
energized by a single phase commercially available 60 Hz, 120 volt
or 240 volt source of alternating current potential, but in
appropriate cases this equipment can be designed for use with other
low frequency, low voltage or d-c sources. Static power converter
12 comprises generally a rectifier 13 and a solid state inverter 14
for converting the unidirectional rectifier output to an ultrasonic
frequency wave for driving an induction heating coil 15. Induction
heating coil 15 is a single layer, annular, flat spiral, air-core
coil would with solid flat strip conductors or braided ribbon
conductors with a rectangular cross section. To generate sufficient
magnetic flux to heat the utensil to the desired cooking
temperature, coil 15 is tightly wound with the short
cross-sectional dimension of the conductors facing upwards and
adjacent turns separated by a flat insulating strip 20.
In the cooking appliance (FIG. 2), induction heating coil 15 is
appropriately mounted in a horizontal position immediately below a
non-metallic support 16 typically made of a thin sheet of glass or
plastic. Support plate 16 is essentially non-conducting but may
have some metallic content for decorative or shielding purposes,
and in the same manner coil 15 is nominally flat and may be
somewhat shaped or configured for several reasons. Support plate 16
is referred to as the cooking surface and supports the metallic
cooking utensil 17 to be heated. Cooking utensil 17 is more
particularly an ordinary cooking pot or pan, a frying pan, or some
other available metallic utensil used in food preparation. The
utensil can be made of a magnetic material such as magnetic
stainless steel, enameled steel, or case iron; a non-magnetic
material such as aluminum; or a laminated product such as
copper-clad stainless steel or triple-clad (stainless steel-cast
iron-stainless steel). Special cooking utensils are not required,
although the best and most efficient results are obtained by
optimizing the size, shape, and material of the cooking utensil.
Magnetic steel utensils couple well to coil 15 and are heated most
efficiently, while some copper-clad utensils and thick aluminum
utensils do not couple well to the coil, and the laminated and cast
iron utensils are inbetween. Any of these may be used, however, in
an induction cooking appliance when the coil 15, static power
conversion circuit 12, and the gap between the coil and utensil are
properly designed. Ordinarily a gap of at least one-eighth inch is
required between the top of coil 15 and the bottom of utensil 17 to
allow space for support 16, and the gap is no greater than about
one-half inch at full power in order to couple sufficient power
into the utensil bottom to produce adequate heating for cooking
purposes. Although an air-core coil 15 is illustrated,
ferromagnetic-core coils can be used also.
Operation of solid state inverter 14 to impress an ultrasonic
frequency wave on induction heating coil 15 results in the
generation of an alternating magnetic field. The magnetic flux is
coupled across the air gap through non-metallic support 16 to
utensil 17. An ultrasonic frequency above 18 kHz or so is generally
considered to be the threshold of human hearing and is selected to
make the cooking appliance inaudible to most people. An essential
feature of the invention is the non-metallic support 16 which, as
shown in FIG. 3, preferably has a relatively smooth and
substantially unbroken utensil supporting surface. At ultrasonic
frequencies there are insignificant reaction forces which at lower
frequencies would cause utensil 17 to move horizontally when placed
on the cooktop surface approximately centered with respect to one
of the induction surface unit positions illustrated in dotted
lines. FIG. 3 shows an induction cooking appliance in the form of
an electric range, with a control knob 21 for each unit on the
upstanding control panel for turning the individual unit on and off
and setting the desired heating level or utensil temperature.
The transfer of energy to utensil 17 to heat it is relatively
efficient since heat is generated only in the utensil and none is
lost because of mismatch in size between coil and utensil. Another
feature of induction cooking is that the surface of support plate
16 is relatively cool since the highest temperatures involved are
about 450.degree.F, the approximate maximum temperature to which
the bottom of utensil 17 is heated to cook food. Because of the
cool cooking surface, spilled foods do not burn and char and hence
both support plate 16 and utensil 17 are easy to clean. The cool,
smooth support also makes it possible to use the surface before
cooking, or even immediately after cooking, for other functions
related to food preparation such as opening cans, trimming and
cutting vegetable, transferring cooked foods from the cooking
utensil to a serving dish, etc. Moreover, the inductive heating of
cooking utensils is relatively uniform and results in a low thermal
mass system. Since there is a relatively low storage of heat in
utensil 17, the heating level or temperature to which the utensil
is heated can be changed rapidly, as from boiling to simmering to
warming temperatures.
The particular form of power converter circuit 12 illustrated by
way of example in FIG. 4 is characterized by a relatively simple
and inexpensive inverter that uses only one thyristor and control
circuit, and employs induction heating coil 15 in a dual function
for coupling power to the utensil (load) and providing commutating
inductance in the thyristor commutation circuit. In the preferred
embodiment to be discussed, power converter input terminals 22 and
23 are energized by a 120 volt, 60 Hz source of a-c supply voltage.
Rectifier 13 is a conventional full wave diode bridge rectifier,
but can be replaced by a phase controlled bridge rectifier when it
is desired to control the power output of the ultrasonic frequency
generator, and therefore the heating level or temperature to which
the utensil is heated, by varying the d-c supply voltage. The full
wave rectified output of bridge rectifier 13 is not applied to the
filter network comprised by a series filter inductor 24' and shunt
filter capacitor 24 until after the closure of an on-off switch 32.
As will be explained later, power is applied to firing control
circuit 33 before energizing inverter circuit 14. Inverter 14 is a
one-tyristor series capacitor commutated or series resonant
inverter that generates opposite polarity damped sinusoidal pulses.
The power circuit comprises essentially induction heating coil 15
connected in series circuit relationship with a commutating
capacitor 27 and a uni-directional conducting thyristor 28 between
d-c supply terminals 25 and 26. A diode 29 to conduct power current
in the reverse direction is connected across the load terminals of
thyristor 28. A series RC snubber circuit 30 is also connected
across the load terminals of thyristor 28 for dv/dt protection to
limit the rate of rise of reapplied forward voltage to the device.
The power circuit also includes a reset inductor 31 connected
directly across commutating capacitor 27. The function of reset
inductor 31 is to reset commutating capacitor 27 between cycles of
operation when both thyristor 28 and diode 29 are non-conductive.
Thyristor 28 is also known as a silicon controlled rectifier, and
although other controlled solid state power devices can be
substituted, the combination of the inverse-parallel connected
silicon controlled rectifier and diode are clearly preferred in
this low cost circuit. Only one gating circuit is required since
diode 29 becomes forward biased and conducts when the current in
the series resonant circuit reverses in the negative polarity half
cycle.
More accurately speaking the inverter load is the loss in the
utensil. Induction heating coil 15 functions as the primary winding
of an air-core transformer, while utensil 17 functions as a
single-turn secondary winding with a series resistance 17r
representing the resistive part of the I.sup.2 R or eddy current
losses, and hysteresis losses where applicable. The currents and
volatges induced in utensil 17 when the induction surface unit is
in operation are determined essentially by the transformer laws.
The physical equivalent for utensil 17 in the form of a single turn
winding and resistive losses 17r is given in FIG. 4. FIG. 5 shows a
simplified equivalent electrical circuit for coil 15 and utensil
17. Coil 15 is represented by a series connected inductance 15i and
resistance 15r and these are in turn in series with the parallel
combination of an inductance 17i and resistance 17r representing
the utensil. This electrical equivalent circuit is based on
conventional transformer equivalent circuit analysis, and has been
found to reasonably agree with experimental results.
With the utensil load in place, the commutating inductance for the
series resonant circuit comprising coil 15 and commutating
capacitor 27 is composed of both the coil inductance 15i and the
reflected utensil inductance 17i. Under no-load conditions, with
the utensil removed from the induction surface unit, the amount of
commutating inductance increases. This causes a change in the
resonant frequency of the series resonant circuit, and there is a
decrease in the inverter output frequency. With an average or
selected utensil load in place, this series resonant circuit is
tuned to resonance at a resonant frequency higher than the highest
desired output frequency. The ultrasonic output frequency range of
interest is between approximately 18 kHz and 40 kHz. The upper
limit of this frequency range is determined largely by economic
considerations, in conjunction with the high frequency limitations
of available thyristor devices.
The operation of the power circuit portion of inverter 14 will be
reviewed briefly before explaining the structure and operation of
firing control circuit 33. It is assumed that firing or gating
control circuit 33 supplies firing signals at a variable repetition
rate to the gate of thyristor 28, to operate the inverter with a
corresponding repetition rate or output frequency according to the
heating level or specific utensil temperature that is desired.
Further details on the operation of the power circuit are given in
the aforementioned Harnden and Kornrumpf application, Ser. No.
200,424. As was previously mentioned, the unit is turned on and off
and the power level is set by means of knob 21 on the range control
panel (FIG. 3). In steady state operation of the inverter, it is
assumed that commutating capacitor 27 is reset negatively
subsequent to the previous complete conduction cycle by reset
inductor 31. The application of a firing pulse to thyristor 28
causes it to turn on, energizing the damped series R-L-C
oscillatory circuit comprising coil 15, commutating capacitor 27,
and the reflected inductance and losses in utensil 17. A positive
polarity half sinusoidal current pulse flows through induction
heating coil 15 and charges commutating capacitor 27 to a value
exceeding the supply voltage V.sub.dc. At the end of the half
cycle, the current drops to zero and then reverses as commutating
capacitor 27 discharges negatively through diode 29. Commutating
capacitor 27 applies a reverse bias to thyristor 28 at the end of
the positive half cycle as the current drops to zero, and turn-off
is assured by the reverse voltage applied to the thyristor by
conducting diode 29. Referring also to FIG. 6a, the current through
induction heating coil 15 at the end of the negative polarity half
cycle remains at zero since the next gating pulse is not applied to
thyristor 28 at this time. While power devices 28 and 29 were
conducting, the current in reset inductor 31 was increasing due to
the net positive d-c voltage on commutating capacitor 27 during the
conduction cycle. During the power circuit off-time reset inductor
31 discharges, thereby leaving commutating capacitor 27 negatively
charged at the end of the circuit off-time. The supplying of a gate
pulse to thyristor 28 begins the next complete cycle of
operation.
FIG. 6a shows in full lines the sinusoidal induction coil current
for two complete cycles of operation separated by a time interval
34 corresponding to the circuit off-time. The corresponding
commutating capacitor voltage under steady-state conditions with
the utensil load in place is shown in full lines in FIG. 6b. At the
end of the conduction cycle capacitor 27 has a negative voltage
lower than the peak positive voltage, and the action of reset
inductor 31 during the time delay interval 34 is to change the
capacitor voltage almost linearly as indicated at 35, leaving the
capacitor more negatively charged at the end of time delay interval
34. Due to this extra charge on commutating capacitor 27, the peak
capacitor voltage 36 during the next cycle of operation is higher,
as compared to a power circuit without reset inductor 31, since the
system energy is replenished. The effect of shortening time delay
interval 34 by increasing the repetition rate of the gating pulses
is to increase the inverter repetition rate or output frequency and
therefore the current and power supplied to utensil 17. Increasing
the inverter output frequency also has the beneficial result of
increasing the amplitude of the sinusoidal current pulses as well
as their frequency. This is illustrated in FIG. 6a by the second
cycle dashed line current waveform. By advancing the thyristor
gating pulse, the raio of conduction time to capacitor recharge
time decreases, thereby on a steady-state basis increasing the
average current in reset inductor 31. The result is that, referring
to the dashed line capacitor voltage characteristic in FIG. 6b,
commutating capacitor 27 is charged negatively to a higher negative
voltage during the circuit off-time as indicated at 35' so that the
peak capacitor voltage 36' during the next conduction cycle is
higher than the peak voltage 36 for the lower inverter frequency
case. A limit on inverter frequency is reached when the value of
the current in reset inductor 31 becomes significant in comparison
to the high frequency reverse current pulse in coil 15 and
commutating capacitor 27, since this in effect reduces the
commutation time available to the thyristor. In summary, there are
two effects that increase the power in watts supplied to utensil 17
when the inverter output frequency is increased. There are larger
and more frequently applied current pulses in induction heating
coil 15. In addition, watts control can be obtained by varying the
input d-c voltage by using a phase controlled rectifier 13 as
previously mentioned. Furthermore, the mechanical technique of
raising and lowering the induction heating coil 15 relative to the
utensil to thereby change the gap spacing can also be used. This is
further explained in Harnden and Kornrumpf application Ser. No.
200,424.
The function of control circuit 33 is not only to generate the
train of variable repetition rate firing pulses for thyristor 28,
but also to incorporate control logic which ensures reliable,
automatic, and satisfactory operation of the induction cooking
appliance. This is done in this control circuit by modifying the
normal operation of the control circuit, specifically the turn-on
circuitry for producing the firing signals that initiate turn-on of
thyristor 28. The firing pulse repetition rate is changed to a
lower more satisfactory value, or the generation of firing pulses
is inhibited completely when certain predetermined abnormal circuit
conditions are sensed that tend to lead to malfunction or failure
of the power device or other power circuit components. In addition
to these protective circuit features, the control circuit is used
during start-up and shut-down of the unit, and to adapt the unit
for operation in the no-load condition by sensing the absence of a
utensil coupled to the induction heating coil and modifying the
operation of the control circuit. The user adjustment potentiometer
40 and output frequency control 41 determine the basic input
voltage level to a voltage controlled pulse generator 46. The
firing pulses produced by voltage controlled pulse generator 46 are
amplified by pulse amplifier 47 and applied to the gate cathode of
thyristor 28. These functional units comprise the turn-on or firing
signal circuit for producing firing pulses at a selected repetition
rate to thereby establish a basic inverter output frequency
corresponding to the heating level or utensil temperature that is
desired. Low voltage d-c power supply 48 is connected between power
rectifier terminals 25' and 26, and establishes a signal level
positive low voltage d-c supply terminal 49 and the common negative
supply terminal 26 between which the various functional units 40-47
of the control circuit are connected.
Maximum frequency limit control circuit 42 is a protective cicuit
feature that prevents the inverter from entering an unstable short
commutation time mode of operation for the reasons given in the
discussion of FIG. 6a. Low input voltage detection circuit 43 has
an input from positive d-c supply terminal 25 and is a protective
circuit feature that is operative upon the reduction of the input
voltage to initially reduce the repetition rate of the firing pulse
generator 46 and, after a predetermined time delay, to lock-out or
inhibit pulse generator 46. When the input voltage is too low to
yield sufficient commutation energy in commutating capacitor 27, as
during a brown-out, there may be a commutation failure. This
circuit is also operative during start-up before filter capacitor
24 is fully charged, and is effective to control starting
transients caused by the interaction of the series resonant power
circuit and the parallel resonant circuit formed by reset inductor
31 and capacitor 27. During shut-down this circuit operates to
remove power from the power circuit in a controlled manner since
the pulse generator continues to operate during the time delay
interval. Utensil presence detection circuit 44 lowers the
repetition rate of the firing pulses developed by voltage
controlled pulse generator 46 to an audible level when the
induction surface unit has no load, that is, when the unit is
turned on before the utensil is in cooking position or when the
utensil is removed before turning off the unit. This is one way to
annunciate an operating unit to the user. The circuit is rendered
operative by the sensing of a power circuit parameter indicative of
the absence of a coupled utensil, in particular the high initial
reapplied forward voltage at the anode of thyristor 28. The
thyristor overvoltage detection circuit 45 is a protection circuit
feature that monitors the thyristor forward blocking voltage and
adjusts the repetition rate of voltage controlled pulse generator
46 to keep the thyristor in a safe operating mode. Due to the fact
that the amplitude of the sinusoidal current pulses in induction
heating coil 15 and the commutating capacitor voltage increases
with an increase in the inverter output frequency, this circuit
lowers the output frequency when the forward blocking voltage
exceeds a predetermined critical value lower than the peak forward
voltage rating of the thyristor. This reduces the forward blocking
voltage applied to the thyristor. Thyristor over-voltage detection
circuit 45 accordingly has an input from the anode of thyristor 28.
It will be recalled that dv/dt protection for the thyristor is
obtained by use of series RC circuit 30.
An ultrasonic frequency static power converter provided with a
firing control circuit 33 with these features operates from no load
to full load with a high degree of reliability. Firing control
circuit 33 is preferably fabricated as a monolithic or hybrid
integrated circuit. In order to avoid the addition of power
consuming components to the inverter power circuit, the trade off
is made between retaining a simple power circuit and increasing the
complexity of the control circuit. Hence the desirability of
manufacturing it in integrated circuit form. Another feature of the
control circuit is that the detection of the absence of the utensil
by sensing a power circuit voltage or other power circuit parameter
in the manner to be explained is in keeping with the principle a
substantially unbroken utensil support 16. Although this could be
done by a mechanical probe projecting through support plate 16 and
extending above the utensil supporting surface, this is not
compatible with a smooth top appliance. It is also desirable to
fabricate the thyristor-diode combination 28, 29 and power
rectifier 13 as hybrid or monolithic integrated circuits. These low
cost approaches are needed especially in consumer-oriented
appliances to reduce the cost as well as to save space.
The components illustrated in the detailed circuit diagram of
firing control circuit 33 shown in FIG. 7 are numbered
consecutively from left to right for the convenience of the reader.
The approach to the design of this circuit is compatible with
monolithic integrated circuit techniques. The low voltage d-c power
supply 48 comprises a pair of voltage dropping resistors 118 and
121 connected in parallel between the high and low voltage positive
d-c terminals 25' and 49. A pair of series Zener diodes 124 and 125
provide a Zener regulated low voltage supply between terminals 49
and 26. Typically the control circuit is supplied with 15 volts d-c
between terminals 49 and 26. The full wave rectified 60 Hz voltage
appearing at the power rectifier terminals 25' and 26 is filtered
by a pair of capacitors 123 and 126, each connected between low
voltage terminals 49 and 26. Filter capacitor 123 is an energy
storage electrolytic capacitor while filter capacitor 126 is a high
frequency ceramic capacitor to provide a low source impedance so
that there is sufficient current to generate a firing pulse of the
required magnitude.
Voltage controlled pulse generator 46 comprises essentially a
complementary unijunction transistor relaxation oscillator. The
circuit includes a timing capacitor 109 connected in series with
the collector-emitter path of an n-p-n control transistor 110 and
an emitter resistor 111 between terminals 49 and 26. The base
terminals of CUJT 113 are respectively connected in series with
base resistors 112 and 114 between terminals 49 and 26, the emitter
of the active device being connected to the negative plate of
capacitor 109. The complementary unijunction transistor is similar
to the ordinary unijunction transistor but operates in the third
quadrant rather than in the first quadrant. The device is further
described in application note No. 90.72, dated Feb. 1968, which is
available from the Semiconductor Products Department, General
Electric Company, Syracuse, New York. Used in an oscillator the
device has excellent frequency stability. Transistor 110 functions
as a variable impedance in the charging circuit for timing
capacitor 109 by virtue of the fact that the collector current is a
function of the base voltage. The d-c voltage level on a control
capacitor 108 connected between the base of transistor 110 and
terminal 26 thus determines the rate of charging of capacitor 109
and hence the pulse generator repetition rate. In operation, timing
capacitor 109 repetitively charges negatively through transistor
110 and emitter resistor 111, and complementary unijunction 113
breaks over and conducts in each charging cycle when the emitter
peak point voltage is reached. Capacitor 109 then discharges
through base resistor 112 and generates a firing pulse that is
amplified by pulse amplifier circuit 47. The repetition rate of
pulse generation depends on the voltage on control capacitor 108,
which is a relatively large capacitor (such as 1 microfarad) so
that the repetition rate ramps from one setting to another as the
cooking temperature or heating level is adjusted or as the
protection circuits operate automatically.
Pulse amplifier 47 comprises a p-n-p transistor amplifier 115
connected in series with voltage divider resistors 116 and 117
between terminals 49 and 26, the base of the active device being
connected directly to the base 1 of CUJT 113. The junction of
resistors 116 and 117 is connected to a transistor amplifier in the
Darlington configuration comprising transistors 119 and 122. The
collectors of both transistors are coupled directly to positive
supply terminal 49, while the emitter of transistor 119 is
connected through an emitter resistor 120 to negative terminal 26
and the emitter of transistor 122 is coupled to the gate of
thyristor 28. The firing pulse supplied by complementary
unijunction 113 is inverted and amplified by transistor 115, and is
further amplified by Darlington transistors 119 and 122. Emitter
resistor 120 assures a rapid turn-off of transistor 112 when the
pulse is completed. Pulse amplifier circuit 47 must supply a
sufficient firing pulse to properly turn on the power thyristor,
and in the case of a GEC139N silicon controlled rectifier, the
pulse must be 1 ampere in 0.1 microsecond or faster to ensure
uniform turn-on of the device. The use of high frequency ceramic
filter capacitor 126, it will be recalled, provides a low source
impedance to supply the required firing pulse.
The operating or output frequency control 41 is basically a
variable resistance divider network for adjusting the d-c voltage
level on control capacitor 108. This circuit comprises a resistor
52 connected in series circuit relationship with user adjustment
potentiometer 40 and another resistor 54 between low voltage supply
terminals 49 and 26. The movable pointer of user adjustment
potentiometer 40 is connected through a resistor 51 to control
capacitor 108, which was previously described as being part of
voltage controlled pulse generator 46. Resistor 51 functions to
isolate potentiometer 40 from control capacitor 108, and it is
useful to the later description to label the junction between these
two components, or more particularly the positive plate of
capacitor 108, as terminal 108p. Adjustment of potentiometer 40
places a variable amount of resistance in series with control
capacitor 108 and is effective to change the voltage on the control
capacitor and consequently the inverter output frequency. The value
of resistor 52 determines, to a first order, the maximum output
frequency that is possible. The value of resistor 54, on the other
hand, determines the minimum possible output frequency.
The actual maximum output frequency limit is set by maximum
frequency control circuit 42. This circuit includes simply a
resistor 56 and a potentiometer 57 connected in series between low
voltage supply terminals 49 and 26, and a diode 55 connected
between control capacitor terminal 108p and the movable pointer of
potentiometer 57. The control operates as a voltage clamp on the
voltage across control capacitor 108. Diode 55 is normally reverse
biased, but when the potential of terminal 108p rises above a
critical voltage as determined by the setting of potentiometer 57,
the diode becomes forward biased and conducts. Diode 55 and the
portion of potentiometer 57 between its pointer and negative supply
terminal 26, normally at ground, is in parallel with control
capacitor 108 so that the voltage across the capacitor is limited
to 0.6 volts (the forward drop of diode 55) above the value set by
potentiometer 57. The effectiveness of this control is limited only
by its input impedance.
Thyristor overvoltage detection circuit 45 comprises a string of
Zener diodes 81 or other suitable voltage sensitive sensors
connected in series with resistors 82-85 between the anode of
thyristor 28 and negative terminal 26. For purposes of
illustration, only a single Zener diode 81 is shown, but in
practice it is expected that a string of devices will be required
to obtain the desired voltage rating. Polycrystalline varistors can
be substituted in place of the Zener diodes. As is well known, the
string of Zener diodes normally does not conduct current but breaks
over and conducts when the voltage applied across the string
exceeds the sum of their individual breakover voltages. In similar
fashion polycrystalline varistors exhibit a voltage regulating
characteristic. Zener diodes and polycrystalline varistors are
commonly known as voltage regulating sensors although they are not
used in this mode for this application. The thyristor overvoltage
detection circuit 45 also includes a transistor 89 having its base
connected to the junction of resistors 84 and 85, its emitter
connected directly to negative supply terminal 26, and its
collector connected through a resistor 88 to control capacitor
terminal 108p. Furthermore, a filter capacitor 80 is connected
across resistors 84 and 85, and a protective diode 86 for
transistor 89 is connected between the junction of resistors 82 and
83 and low voltage supply terminal 49.
In operation, the application of an overvoltage to the anode of
thyristor 28 causes Zener diodes 81 to break over and conduct
current, and the resulting voltage drop across resistor 85 turns on
transistor 89. Control transistor 89 then tends to discharge
capacitor 108 through resistor 88, which lowers the repetition rate
of pulse generator 46 and results in changing the anode voltage to
a safe level. Resistors 84 and 85 in this circuit set the level of
overvoltage detection and provide a positive turn-off bias for
transistor 89 when the condition has passed and Zener diodes 81 no
longer conduct. Resistor 83 and capacitor 80 comprise low pass
filter which is used to eliminate any capacitive component of
rising thyristor anode voltage passed by the Zener string. Until
capacitor 80 charges to a predetermined voltage, there is by design
insufficient voltage across resistors 84 and 85 to supply the
required base drive current for transistor 89. A high frequency
transient overvoltage fails to charge capacitor 80 to this level.
Diode 86 becomes conductive under appropriate conditions to prevent
the application of an over-voltage to transistor 89. The proper
choice of discharge resistor 88 in this circuit assures the correct
amount of override or takeover of output frequency control 41. A
large resistance value prevents effective control because the gain
is too low, while a small value can increase the gain to an
unstable point.
The principle of operation of utensil detection circuit 44 is that
with no utensil load coupled to induction heating coil 15, the
inverter power circuit (see FIG. 4) is essentially an undamped
oscillatory L-C circuit since there is little loss in the high
frequency resonant circuit. This means that commutating capacitor
27 (before the additional charge supplied by reset inductor 31)
charges negatively as far as it does positively. In the loaded case
(see FIG. 6b) the peak negative capacitor voltage at the end of the
conduction cycle is considerably less than the peak positive
capacitor voltage. Conversely looking at the thyristor forward
voltage at the anode of the device as shown in FIG. 6c, the initial
reapplied forward voltage for the no-load situation illustrated in
full lines during the interval t.sub.1 is characteristically higher
than for the loaded situation shown in dashed lines. The induction
coil current is drawn in dot-dash lines for reference purposes.
Utensil presence detection circuit 44 recognizes the initial value
only of the reapplied thyristor anode voltage, and is operative in
response to the sensing of a predetermined voltage level indicative
of the absence of a coupled utensil to reduce the inverter output
frequency to a safe, low frequency. A portion of this circuit is
identical or similar to the thyristor overvoltage detection circuit
45 just described. These are Zener diode string 71 (only one is
illustrated), resistors 72-75, low pass filter capacitor 70, and
resistor 78 and transistor 79. A variable resistor 77 is
additionally connected in series with resistor 78 and the
collector-emitter path of transistor 79 between terminals 108p and
26. A predetermined relatively high voltage, less than needed to
actuate Zener diode string 81, causes Zener diode string 71 to
break over and conduct, as a result of which transistor 79 turns on
and tends to discharge control capacitor 108. A monostable
multivibrator arrangement is used whereby transistor 79 and the
other discharge circuit components are enabled and can become
conductive only during the relatively short time interval t.sub.1
(such as 4 microseconds) after the appearance of an initial high
thyristor anode voltage.
The monostable timing circuit includes a first p-n-p transistor 63
connected in series with a collector resistor 64 between terminals
49 and 26, and a second n-p-n transistor 68 having its emitter
connected to terminal 26, its collector connected to the base of
transistor 79, and its own base connected through resistors 66 and
67 to low voltage supply terminal 49. The collector of transistor
63 is coupled by means of capacitor 65 to the base of transistor
68. The biasing circuit for transistor 63 is provided by resistor
62. Resistors 58 and 59 are connected in series between the anode
of thyristor 28 and negative terminal 26, and a capacitor 60 is
connected to the junction of these two resistors and to the base of
transistor 63. Normally transistor 63 is biased to the conducting
condition by the circuit completed through resistor 62 to negative
terminal 26, and in similar manner transistor 68 is biased to the
conducting condition by resistors 66 and 67. Capacitor 65 therefore
is charged to approximately 15 volts. A diode 61, connected to the
base of transistor 63 and in series with resistor 62 between
terminals 49 and 26, protects transistor 63 from excessive reverse
voltage. When the thyristor anode voltage goes positive in each
cycle due to the reapplied forward voltage, a reverse bias is to
the base of transistor 63 through resistor 58 and capacitor 60, and
turns off. Capacitor 65 is still charged, however, and due to the
fact that the positive plate is now coupled to negative terminal 26
through resistor 64, the base of transistor 68 is driven sharply
negative, thereby reverse biasing transistor 68 and turning it off.
During the ensuing interval while capacitor 65 discharges and
before transistor 68 turns back on, the overvoltage detection
circuit comprising components 70-79 is free to operate. As soon as
transistor 68 turns off, capacitor 65 discharges through resistor
64, 66, and 67, and at the end of the interval t.sub.1 transistor
68 is no longer reverse biased and again turns on.
By way of brief summary of the operation of the utensil presence
detection circuit 44, a relatively high thyristor anode voltage is
detected during the interval t.sub.1 of each complete cycle by the
circuit comprising components 69-79. The Zener diodes 71 normally
do not conduct, but supply current to series resistors 72-75 when
their combined breakdown voltage is exceeded. Capacitors 69 and 70
prevent actuation of this portion of the circuit by high frequency
transient components of the anode voltage. Transistor 79 is forward
biased by the voltage appearing across resistor 75, thereby tending
to discharge control capacitor 108 in each cycle through the series
discharge circuit comprising resistors 77 and 78 and transistor 79.
This arrangement overrides output frequency control 41 and reduces
the inverter output frequency to the audio frequency range where it
can be heard by the user. The circuit comprising components 58-68
is essentially a monostable multivibrator that allows transistor 79
to turn on in response to a sensed high anode voltage only during
the short time interval t.sub.1 following the reapplication of
positive thyristor anode voltage. Transistors 63 and 68 are
normally conductive and since the collector of transistor 68 is
connected directly to the base of transistor 79, this means that
transistor 79 is clamped off. The appearance of positive anode
voltage in each cycle reverse biases transistor 63 through
resistors 58 and capacitor 60, as a result of which charged
capacitor 65 also applies a reverse bias voltage to transistor 68,
turning it off. Transistor 68 remains nonconducting only during the
interval when capacitor 65 is discharging through resistors 64, 66,
and 67, and again turns on after this short interval t.sub.1 to
clamp the base of transistor 69 so that it is either turned off or
cannot conduct. In actual practice the placing of a utensil on
support 16 and its removal takes a relatively long time as compared
to the period of an ultrasonic frequency cycle. Therefore the
repetition rate of firing pulse generation ramps slowly from the
ultrasonic range to the audio range or vice versa.
An alternative procedure in the event the audible annunciation is
not desired is to simply reduce the output frequency to the lower
limit (18 kHz). Still another technique is to inhibit the inverter
completely and test periodically for a coupled utensil so long as
switch 21 is closed.
Low input voltage detection circuit 43 monitors the voltage on
power circuit positive d-c supply terminal 25 and prevents
thyristor 28 from being fired while there is insufficient
commutating energy in commutation capacitor 27 to ensure proper
circuit operation. This circuit reduces the firing pulse repetition
rate and therefore the inverter output frequency to a low value and
then inhibits further firing pulses until adequate d-c voltage is
returned. Upon initially turning on the unit, the circuit starts
the power circuit with a safe, low operating frequency which is
increased in a safe controlled manner to prevent low commutation
time conditions from occurring. A low input d-c voltage level below
a predetermined voltage is sensed by a Zener diode 90 or other
voltage sensitive or voltage responsive sensor. Under normal
conditions Zener diode 90 is conductive and supplies current to the
remainder of the detection circuit, and it is the absence of this
current that triggers the operation of the circuit. Zener diode 90
is connected in series with a pair of resistors 91 and 92 between
high voltage d-c supply terminals 25 and 26. The junction of these
two resistors is connected through a resistor 93 to the base of a
normally non-conducting p-n-p transistor 97. The emitter of
transistor 97 is connected directly to supply terminal 49, while
the collector is connected through resistor 98 to the base of a
second normally non-conducting n-p-n transistor 95. To establish a
feedback path between transistors 97 and 95, the collector of
transistor 95 is coupled through a resistor 94 to the junction of
resistors 91 and 92. An override or takeover circuit shunting
control capacitor 108 comprises a diode 95a and adjustable resistor
95b connected in series between terminal 108p and the collector of
transistor 95. The voltage in the override circuit is clamped at a
predetermined level by a Zener diode 96 connected between the
emitter of transistor 95 and supply terminal 26.
Transistor 97 is normally biased non-conducting by the current flow
through Zener diode 90 which establishes a reverse biasing
potential at the base of transistor 97, as a result of which there
is no base drive for transistor 95 which is also non-conducting.
Upon the sensing of a low input power circuit d-c voltage, Zener
diode 90 now ceases to conduct and base drive current for
transistor 97 is established by the connection of the base to
negative supply terminal 26 through resistors 93 and 92. Transistor
95 also turns on in snap action fashion by means of the feedback
connection established by resistors 98 and 94. The voltage across
control capacitor 108 is now clamped by conducting diode 95a and
transistor 95 to the voltage of Zener diode 96. This voltage level
sets the basic inverter output frequency to the lower frequency
limit, typically 18-20 kHz. At this frequency, it would be safe to
start gating thyristor 28 at any voltage above the lower limits set
by Zener diode 90.
A low voltage condition in the power circuit not only requires a
low firing pulse repetition rate, but that the firing of thyristor
28 be halted completely. This is accomplished by clamping the
voltage across main timing capacitor 109 in the voltage controlled
pulse generator 46, but this can occur only when transistors 97 and
95 are conducting. To this end, the collector of transistor 97 is
also connected to a resistor 100 which is in series with Zener
diode 102 and a resistor 103. A timing delay capacitor 99 and a
capacitor discharge resistor 101 are both in parallel with the
series connected Zener diode 102 and resistor 103, and the junction
of these last two components is connected to the base of an n-p-n
transistor 106. The collector-emitter of transistor 106 is
connected in series with a pair of voltage divider resistors 105
and 104 between terminals 49 and 26, and the junction of resistors
104 and 105 is connected to the base of a p-n-p clamping transistor
107. The emitter and collector of clamping transistor 107 are
connected directly to the terminals of main timing capacitor 109.
With this arrangement, it is seen that the turning on of transistor
106 supplies base drive current to transistor 107, which also turns
on to clamp the complementary unijunction pulse generator.
The change of state of transistor 97 from non-conducting to
conducting when a low input voltage is sensed also initiates
current flow through the low pass filter comprising resistor 100
and time delay capacitor 99, which charges at a rate dependent upon
the time constant of these two components. After a predetermined
time delay, capacitor 99 has charged to a voltage level sufficient
to cause Zener diode 102 to break over and conduct. Accordingly,
after this time delay base drive current is supplied to transistor
106, which turns on and sequentially results rendering conductive
clamping transistor 107. The entire sequence of events, then, is
that transistors 97 and 95 initially become conductive and clamp
the voltage across control capacitor 108 to the level determined by
Zener diode 96, which sets the pulse generator firing repetition
rate to its lower frequency limit. After a time delay determined by
the time constant of resistor 100 and capacitor 99, transistors 106
and 107 also turn on to completely inhibit the generation of firing
pulses. Upon the return of sufficient input d-c voltage to enable
safe operation of the power circuit, Zener diode 90 conducts
causing transistors 97 and 95 to turn off initially, as well as
transistors 106 and 107 upon the discharge of capacitor 99 through
discharge resistor 101. Control circuit 33 then starts generating
firing pulses at a safe minimum repetition rate and increases the
repetition rate to obtain the desired inverter output frequency at
a rate determined by the time constant of control capacitor 108 and
the control impedance of output frequency control circuit 41.
Low input voltage detection circuit 43 also operates during
shut-down of the induction surface unit when switch 21 (FIG. 4) is
turned off to remove power from the unit. The main power supply
filter capacitor 24 (FIG. 4) of course requires a few 60 Hz cycles
to discharge. As soon as the input voltage sensed by Zener diode 90
drops to a low enough value, transistors 97 and 95 turn on to clamp
control capacitor 108 to a voltage resulting in a low repetition
rate. At the end of the time delay, clamping transistor conducts
and inhibits further generation of firing pulses. This controlled
deenergization of the power circuit controls the transients in the
system including those caused by the interaction of the series
resonant circuit with the parallel resonant circuit formed by
capacitor 27 and reset inductor 31, which could cause a commutation
failure.
By way of example of a specific induction surface unit suitable for
use in an electric range with 120 volt service, the required
maximum power output is 1500 watts. The peak input d-c voltage to
the power circuit is 150 volts, and signal level control circuit 33
is supplied with 15 volts d-c. The ultrasonic frequency range of
interest is about 18 kHz to 40 kHz. The combined equivalent coil
inductance 15i and utensil inductance 17i (FIG. 5) is typically 150
microhenries, and the combined equivalent coil resistance 15r and
utensil resistance 17r is typically 15 ohms. Reset inductor 31 is,
of course, relatively large. Using a power thyristor 28 with a peak
forward voltage of 800 volts, Zener diodes 81 detect an overvoltage
at the anode of the thyristor of 750 volts or possibly less. Zener
diodes 71 in utensil presence detection circuit 44 detect an
initial reapplied forward voltage at the thyristor anode of about
400 volts. The critical low input voltage that actuates low input
voltage detection circuit 43 is typically 90 volts. For a
counter-top warming or cooking appliance a lower maximum power
output is satisfactory, such as 200-400 watts.
The multi-purpose control circuit 33 can be used with a variety of
inverter configurations employing other power semiconductors such
as the diac and triac. With the possible exception of the utensil
presence detection circuit, the remainder of the control circuit is
suitable for usage with inverters in general for applications other
than in cooking appliances. Other one-thryistor, two-thyristor,
one-transistor, and two-transistor inverters especially appropriate
for ultrasonic frequency induction cooking appliances are described
in the aforementioned application Ser. No. 200,424, and also in
application Ser. No. 200,526 by David L. Bowers, Donald S.
Heidtmann, and and John D. Harnden, Jr., filed Nov. 19, 1971, and
assigned to the same assignee as this invention. A similar approach
is used to determine the exact functions and configurations of the
protection circuits. Some of these inverters require two firing or
base drive circuits, so that two of the control circuits 33 may be
required, one for each device, although it may not be necessary to
include all of the protective features in each control circuit.
While the invention has been particularly shown and described with
reference to a preferred embodiment therof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
* * * * *